1. Field of the Invention
The present invention relates to a method for performing burn-in test which is a current-carrying test effective for screening target elements. The present invention especially relates to a burn-in test applied to a light source unit that includes a light source for thermally-assisted magnetic recording, and to a test apparatus for performing the burn-in test.
2. Description of the Related Art
With the explosion in the use of the Internet in these years, a huge amount of data that are incommensurably larger than ever are stored and used on computers such as servers and information-processing terminals. This trend is expected to further grow at an accelerated rate. Under these circumstances, demand for magnetic recording apparatuses such as magnetic disk apparatuses as mass storage is growing, and the demand for higher recording densities of the magnetic recording apparatuses is also escalating.
In the magnetic recording technology, it is necessary for magnetic heads to write smaller recording bits on magnetic recording media in order to achieve higher recording densities. In order to stably form smaller recording bits, perpendicular magnetic recording technology has been commercially implemented in which components of magnetization perpendicular to the surface of a medium are used as recording bits. In addition, thermally-assisted magnetic recording technology that enables the use of magnetic recording media having higher thermal stability of magnetization is being actively developed.
In the thermally-assisted magnetic recording technology, a magnetic recording medium formed of a magnetic material with a large energy KU is used so as to stabilize the magnetization, then anisotropic magnetic field of a portion of the medium, where data is to be written, is reduced by heating the portion; just after that, writing is performed by applying write field to the heated portion. Actually, there has been generally used a method in which a magnetic recording medium is irradiated and thus heated with a light such as near-field light (NF-light). In this case, it is significantly important where and how a light source with a sufficiently high light output should be disposed inside a head in order to stably supply a light with a sufficiently high intensity at a desired position on the magnetic recording medium.
As for the setting of the light source, for example, U.S. Pat. No. 7,538,978 B2 discloses a configuration in which a laser unit including a laser diode is mounted on the back surface of a slider, and US Patent Publication No. 2008/0056073 A1 discloses a configuration in which a structure of a laser diode element with a monolithically integrated reflection mirror is mounted on the back surface of a slider.
The present inventors propose a thermally-assisted magnetic recording head with a “composite slider structure” which is constituted by joining a light source unit provided with a light source to the end surface (back surface) of a slider provided with a write head element, the end surface being opposite to the opposed-to-medium surface of the slider. The “composite slider structure” is disclosed in, for example, US Patent Publication No. 2008/043360 A1 and US Patent Publication No. 2009/052078 A1. The advantages of the thermally-assisted magnetic recording head with the “composite slider structure” are as follows:
a) The head has an affinity with the conventional manufacturing method of thin-film magnetic heads because the opposed-to-medium surface and the element-integration surface are perpendicular to each other in the slider;
b) The light source can avoid suffering mechanical shock directly during operation because the light source is provided far from the opposed-to-medium surface; and
c) The head can be manufactured with reduced man-hour and at low cost, because of no need to provide the head with optical components such as a lens or prism which are required to have much high accuracy, or with optical elements having a special structure for connecting optical fibers or the like.
Furthermore, in the “composite slider structure”, the following can be singled out for special mention with respect to characteristic evaluation and reliability evaluation in the manufacturing process:
d) The light source such as a laser diode and the head elements can be evaluated independently of each other; thus the degradation of manufacturing yield for obtaining the whole head can be avoided; whereas, in the case that all the light source and head elements are provided within the slider, the manufacturing yield rate for obtaining the whole head is likely to decrease significantly due to the multiplication of the process yield for the light-source and the process yield for the head elements.
Here, in order to evaluate the reliability of a light source unit including a light source, in particular, a laser diode, burn-in test is an effective way for the evaluation. The burn-in test involves passing an electric current through devices under test (laser diode herein) to measure and evaluate change with time in a characteristic of the devices under test in a conduction state at high temperatures, thereby screening out defective devices. However, the burn-in test takes very long time, for example several to several tens of hours, per laser diode. In addressing this inefficiency, it is very effective to simultaneously evaluate multiple laser diodes at a bar-level before being cut into individual light-source-unit chips in a light source unit manufacturing process. With this parallel operation, burn-in test can be performed on many laser diodes at a time, thereby the number of man-hours and time required to perform the evaluation step can be significantly reduced.
However, it is extremely difficult to bring power-supply probes into contact with an enormous number of electrodes for multiple laser diodes provided on a unit bar at a time. In practice, when metal needles, which are commonly used, are used as the probes, a probe card on which a large number of needles, for example several hundred needles are provided upright needs to be provided for burn-in test of a single unit bar. However, the needles are expensive and, in addition, if a single needle on the card fails to make contact, all needles on the card need to be replaced, which is very uneconomical. Furthermore, when needles are brought into contact with upper electrodes of laser diodes, excessive mechanical stress can be applied to the laser diodes.
When a characteristic of multiple magnetic head elements arranged on a slider bar is evaluated, the needles described above can be used as probes. Such evaluation of a characteristic of magnetic head elements requires only a small amount of measurement time which is on the order of seconds or less per magnetic head element. Therefore, a required number needles for evaluation of one magnetic head element can be used as probes to successively evaluate a characteristic of the individual magnetic head elements on a slider bar in sequence.
It may be contemplated to use sheet-type probes, which are less expensive than needle-type probes, instead of using the needles described above, to perform burn-in test on a unit bar. However, it is very difficult to stably supply power with individual electrodes for laser diodes with sheet-type probes.
In practice, in a unit bar in which laser diodes are provided, electrodes with which sheet-type probes are to make contact are on both of the laser diodes and unit substrates.
That is, multiple electrodes are provided in positions at different heights. The difference in height between an electrode on a laser diode and an electrode on a unit substrate is exactly equal to the height of the laser diode, which is on the order of 10 micrometers (μm) or greater. The quality of contact between a sheet-type probe and an electrode depends in large part on the angle at which the tip of the sheet-type probe makes contact with the electrode. Accordingly, there has been a problem that when sheet-type probes are brought into contact with an electrode on a laser diode and an electrode on a unit substrate which significantly differs in height, the angle at which the sheet-type probes makes contact with the electrode on the laser diode is relatively shallow and therefore the contact between the electrode on the laser diode and the sheet-type probe becomes unstable.
A laser diode is preferably placed so that its p-electrode side faces down (faces the unit substrate) in order to provide more effective heat dissipation. In a laser diode, the active layer which generates the most heat is located closer to the p-electrode side. By placing the laser diode so that its p-electrode side faces down, the active layer is positioned closer to the unit substrate. As a result, the unit substrate can be made more effectively function as a heat sink. However, the electrode on the laser diode in this case is an n-electrode. Typically, the surface of the n-electrode is highly smoothed in order to allow stable wire-bonding required for the laser diode to be mounted in a can package which houses and protects the laser diode. Therefore, when a sheet-type probe makes contact with the electrode on the laser diode at a shallow angle, the contact between the sheet-type probe and the electrode is unstable.
As can be seen from the foregoing, there is a great demand for development of a method of burn-in test that can be efficiently performed on a light source unit constituting a thermally-assisted magnetic recording head having a “composite slider structure” at low cost.